Bleeding in pregnancy. By day now

My wife period generally comes on 17th each moth but last month it delayed, we done pregnancy test on 26th feb it was positive and after 3 days my wife got spotting bleeding not like periods but spotting due to this we again tested her and it come negative 😔 the we done songraphy on same day it was showing that still their are high chances of pregnancy doctor consulted doctor said lets wait for week then we waited for week bleeding did not stop ( spotting) the again on 6th March we visited doctor to check up again doctor done pregnancy test and it came positive again doctor said lets wait till 17th March and do again songraphy hence we are waiting my questions is why bleeding (spotting) happening it's been more than 10 days now worried 😔 our all hopes are decreasing day by day now 😔😔😔😔